23.6 Protozoan Infections of the Urogenital System

Protozoan Infections of the Urogenital System

Trichomoniasis is the most common curable sexually transmitted infection worldwide, with an estimated 156 million new cases per year globally. It's caused by the flagellated protozoan Trichomonas vaginalis, which colonizes the urogenital tract and triggers inflammation. Understanding how this organism causes disease, how to diagnose it, and how to treat it is central to managing urogenital infections in clinical microbiology.

Trichomoniasis Pathogenesis and Effects

Trichomonas vaginalis is an obligate parasite with no cyst stage. It exists only as a motile trophozoite and is transmitted primarily through vaginal intercourse. Because it can't survive long outside the human body, non-sexual transmission is extremely rare.

Once in the urogenital tract, T. vaginalis adheres to epithelial cells using surface adhesins. It then secretes proteases and other cytolytic enzymes that directly damage the epithelial lining. This tissue damage triggers an inflammatory response, recruiting neutrophils and other immune cells to the site of infection.

In females:

• The vagina and cervix are the primary sites of infection
• Inflammation can cause vaginitis and cervicitis
• Chronic infection may contribute to pelvic inflammatory disease (PID)
• Infection disrupts the vaginal microbiome and damages the mucosal barrier, which increases susceptibility to HIV acquisition and transmission

In males:

• The urethra is the primary site, though the prostate and epididymis can also be affected
• Can cause urethritis, prostatitis, or epididymitis
• Most male infections are asymptomatic or produce only mild symptoms, which means infected men often serve as undiagnosed reservoirs of transmission

Symptoms and Diagnosis of Trichomoniasis

Symptoms differ significantly between sexes. Women tend to present with more obvious clinical signs, while men are frequently asymptomatic.

Female symptoms:

• Frothy, yellow-green vaginal discharge with a strong, unpleasant odor
• Vaginal itching, burning, and irritation
• Dysuria (painful urination) and dyspareunia (painful intercourse)
• Lower abdominal pain or discomfort
• "Strawberry cervix" (punctate hemorrhages on the cervix), visible on colposcopy in some cases

Male symptoms:

• Often asymptomatic or very mild
• Urethral discharge, itching, or burning
• Dysuria and increased urinary frequency

Diagnostic methods:

• Wet mount microscopy is the classic approach. A drop of vaginal or urethral discharge is placed on a slide with saline, and you look for motile, pear-shaped trophozoites with their characteristic jerky movement. It's fast and cheap, but sensitivity is only about 60–70% because organisms may be sparse or lose motility quickly after collection.
• Culture on specialized media (such as Diamond's or InPouch TV) is more sensitive than wet mount. Samples are incubated for 2–7 days and then examined for growth. Culture was long considered the gold standard, but it has been largely replaced by molecular methods.
• Nucleic acid amplification tests (NAATs) detect T. vaginalis DNA using PCR or transcription-mediated amplification. These are now the gold standard, with sensitivity above 95% and high specificity. They can be performed on vaginal swabs, urine, or endocervical specimens.
• Rapid antigen detection tests use immunochromatographic assays to detect T. vaginalis antigens. These provide results in minutes and have better sensitivity than wet mount (about 80–90%), making them a useful point-of-care option.

Treatment Options for Trichomoniasis

Both first-line drugs are 5-nitroimidazoles, which work by generating toxic free radicals inside the anaerobic protozoan, damaging its DNA.

Metronidazole (Flagyl) is the standard first-line treatment:

1. Single-dose regimen: 2 g metronidazole taken orally at once. Cure rates are approximately 90–95%.
2. Multi-dose regimen: 500 mg metronidazole taken orally twice daily for 7 days. Cure rates reach 95–100%. The CDC now recommends this 7-day regimen for women, as recent data show improved cure rates compared to single dose.

Tinidazole (Tindamax) is an alternative:

• Single-dose oral therapy: 2 g tinidazole
• Cure rates of 86–100%, comparable to metronidazole
• May be better tolerated with fewer gastrointestinal side effects

Partner therapy is essential. All sexual partners must be treated simultaneously, regardless of whether they have symptoms. The same regimens are used. Without partner treatment, reinfection rates are high.

Follow-up testing is recommended at 2 weeks to 3 months after treatment to catch treatment failure or reinfection. NAATs should not be performed earlier than 2 weeks post-treatment to avoid false positives from residual DNA.

Drug resistance to metronidazole is uncommon but has been documented. For resistant cases, higher doses or extended treatment durations may be tried. Tinidazole sometimes works when metronidazole fails. In refractory cases, intravaginal paromomycin has been used, though evidence is limited.

Considerations for Special Populations

Pregnancy: Trichomoniasis during pregnancy is associated with preterm birth, premature rupture of membranes, and low birth weight. Metronidazole can be used in pregnant patients (it's considered safe across all trimesters), but tinidazole is not recommended during pregnancy due to insufficient safety data. Symptomatic pregnant women should be treated with the 7-day metronidazole regimen.

Immunocompromised patients, particularly those with HIV, may experience more severe or persistent symptoms. Coinfection with T. vaginalis increases HIV viral shedding in the genital tract, which raises transmission risk. These patients may require longer treatment courses and closer follow-up.

Adolescents should be screened and treated using the same protocols as adults. Confidential STI screening and partner notification are particularly important in this population.

Antibiotic resistance in T. vaginalis remains relatively rare but is a growing concern. Ongoing surveillance and research into alternative antiprotozoal agents are needed to stay ahead of resistance trends.